As far as the Bush administration is concerned, it was never a question of whether they would withdraw from the Anti-Ballistic Missile (ABM) treaty - but when.

Administration spokesmen, along with the president himself, have made no secret of their view that the treaty is a product of old Cold War thinking.

Abandoning a treaty is a major step... The key question is how Russia will respond

It is simply no longer relevant and, worse they say, its existence actually hinders the US from developing effective defences against a limited ballistic missile attack.

Now the Bush administration is signalling that the formal announcement of its decision to withdraw from the agreement is imminent.

Washington must give Moscow six months' notice of its intention.

Domestic critics of the Bush missile defence plans will react strongly to the news.

The Russian factor

Their view is that the terrorist attacks of 11 September show that the real threats to US security do not come from a tiny number of missile-armed states.

11 September: Turning point

But 11 September appears to have convinced the Bush team that it is essential to develop anti-missile defences.

Abandoning a treaty is a major step. And, of course the key question is how Russia will respond.

The Americans have been negotiating hard with Moscow to try to get some relaxation of the ABM treaty's terms or at least some sort of tacit approval from the Russians for America's defence plans.

Such an understanding has not been possible. Perhaps one might be reached within the next six months.

Many inside the Bush administration believe that the Russian President Vladimir Putin knows full well that the Bush team is set on developing missile defences and, equally, that he understands the limited scope of what is being proposed.

But Mr Putin's efforts to forge a new relationship with Washington could be undermined.

His domestic critics in Russia will seize on the abandonment of the ABM treaty as a sure sign that Washington cannot be trusted.